There are different types of semiconductor packages for different electronic products such as Ball Grid Array, BGA, having a plurality of ball terminals such as solder balls disposed at the bottoms of the semiconductor packages. Normally, the ball terminals should be formed on an exposed surface of a substrate by reflowing with sufficient numbers to be I/O connecting terminals for the semiconductor packages to electrically connect to an external printed circuit board for operations and to meet the high-density SMT requirements. During semiconductor packaging processes, substrates experience different heat treatments such as curing of die-attaching materials and encapsulant and reflowing of ball terminals. Furthermore, when a semiconductor package is under operations or under thermal cycle test, TCT, thermal stresses will generate between the semiconductor package and the external printed circuit board due to mismatch of the coefficients of thermal expansion, CTE, where the thermal stresses will exert on the ball terminals, especially on those located at the peripheries or corners of the substrate and close to the edges of IC chips causing substrate warpage and/or solder ball cracks leading to poor product reliability. Moreover, the ball terminals located at the peripheries or corners of the substrate are easily experienced the impact forces during a drop test As shown in FIG. 1, a conventional semiconductor package 100 is a window-type BGA package, primarily comprising a substrate 110, a die-attaching material 120, a chip 130, a first row of ball terminals 140, and a second row of ball terminals 150. The substrate 110 has a covered surface 111, an exposed surface 112, and a slot 115 as a wire-bonding window. The exposed surface 112 is exposed from an encapsulant 170 for SMT. A solder mask 117 is formed on the exposed surface 112 where a plurality of internal bonding fingers 116 and a plurality of external ball pads 118 are exposed from the solder mask 117 for bonding a plurality of electrical connecting components 160 and the ball terminals 140 and 150. The die-attaching material 120 is formed on the covered surface 111 of the substrate 110 to firmly attach the chip 130 onto the substrate 110. A plurality of bonding pads 132 are disposed on the active surface of the chip 130 where the electrical connecting components 160 such as bonding wires formed by wire bonding pass through the slot 115 to electrically connect the bonding pads 132 of the chip 130 to the internal bonding fingers 116 of the substrate 110. The encapsulant 170 is formed on the covered surface 111 of the substrate 110 and in the slot 115 by molding to encapsulate the chip 130 and the electrical connecting components 160.
The first row of ball terminals 140 and the second row of ball terminals 150 are disposed on the external ball pads 118 on the exposed surface 112 of the substrate 110 with the second row of ball terminals 150 relatively further away from the slot 115 than the first row of ball terminals 140. Therefore, the distance from the neutral point, DNP, to the second row of ball terminals 150 is greater than the one of the first row of ball terminals 140, i.e., the second row of ball terminals 150 are adjacent to the peripheries or corners of the substrate 110 and easily experience the concentrated stresses.
However, heat will generate during semiconductor packaging processes, such as curing of die-attaching materials 120 and the encapsulant 170, reflowing of the ball terminals 140 and 150, and the thermal cycle test, or the operation of the semiconductor packages, causing thermal stresses due to CTE mismatch of different materials exerted on the second row of ball terminals 150 leading to solder ball cracks. Moreover, the impact forces during drop tests will also exert on the second row of ball terminals 150 leading to ball drop or ball crack. The quality of electrical connections will be greatly affected by the solder ball cracks or dropped balls. Furthermore, some of the ball terminals 150 located adjacent to the corners of the chip 130 will also have solder ball cracks and ball drop issues due to substrate warpage. Moreover, the die-attaching material 120 becomes low viscosity and easily flowing during die-attaching processes with raised temperatures and exerted pressure leading to bleeding or creeping of the die-attaching material 120.